1. Field of the Invention
The present invention relates to a seal for a rotating shaft of a turbocharger. More particularly, the present invention relates to a seal with an inner surface having a plurality of striations extending in a helical path about a rotating shaft of a turbocharger.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft is rotatably supported in the bearing housing and couples the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it compresses ambient air entering the compressor housing, thereby increasing the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
The shaft coupling the turbine wheel and compressor impeller extends into the turbine housing through a turbine side of the bearing housing. The shaft also extends into the compressor housing through a compressor side of the bearing housing. One or more piston rings are typically used as a seal between the shaft and the turbine side of the bearing housing to prevent lubricating oil within the bearing housing from migrating into the exhaust gas in the turbine housing. Similarly, one or more piston rings are also typically used as a seal between the shaft and the compressor side of the bearing housing to prevent lubricating oil within the bearing housing from migrating into the compressed air in the compressor housing. The piston rings also prevent the exhaust gas and compressed air from migrating into the bearing housing.
The piston rings act as a labyrinth seal. Labyrinth seals are a type of mechanical seal which are not fluid-tight but limit leakage by means of providing a tortuous path through which a substance, i.e., lubricating oil, must flow. In addition to the piston rings on the turbine side, a pressure in the turbine housing is generally greater than a pressure in the bearing housing such that lubricating oil does not tend to leak out of the bearing housing past the piston rings and into the exhaust gas in the turbine housing. Likewise, in addition to the piston rings on the compressor side, a pressure in the compressor housing is generally greater than a pressure in the bearing housing such that lubricating oil does not tend to leak out of the bearing housing past the piston rings and into the compressed air in the compressor housing.
Since the pistons rings are not fluid-tight, the pistons rings may not entirely prevent lubricating oil from leaking into the turbine housing and/or the compressor housing. For example, under some engine operating conditions, the pressure in the turbine housing is less than the pressure in the bearing housing, which may allow lubricating oil to leak past the piston rings on the turbine side into the turbine housing. Lubricating oil that leaks past the piston rings on the turbine side is usually burned by the exhaust gas and is discharged with the exhaust gas as blue smoke. Similarly, under some engine operating conditions, the pressure in the compressor housing is less than the pressure in the bearing housing, which may allow lubricating oil to leak past the piston rings on the compressor side into the compressor housing. Lubricating oil that leaks past the piston rings on the compressor side may accumulate in the intake system or mix with the incoming air and be burned during the combustion process, thereby hindering engine performance. In addition, burning oil during the combustion process leads directly to increased emissions.
It is desirable, therefore, to minimize the amount of lubricating oil leaking into the turbine housing and/or the compressor housing. As such, it is desirable to provide improved seals between the shaft and the bearing housing.